Differences between the T400 and the X200 apart from display size?

T400 has optical drive bay, optional discrete graphics, and optional PCMCIA slot/SD card reader/smart card reader (in addition to the standard Express Card slot, depending on model type).

If you want to buy a T400, those with WXGA+ (1440*900) screen and integrated graphics are recommended. (The optional ATI HD3470 discrete graphics is very weak and useless.)

There is a usable Atheros card for X200, T400 and W500 series. Atheros AR2425 with FRU number 43Y6511.